1. Field of the Invention
The present invention relates generally to techniques for calibrating an adaptive antenna array system, and more particularly to a method and apparatus for calibrating a multi-carrier smart antenna array system.
2. Description of the Prior Art
Antenna arrays are commonly used in a wide variety of systems that transmit and/or receive radio frequency (RF) signals. Examples of such systems include wireless communication systems, such as cellular telephone systems, and radar systems. An antenna array, which includes a plurality of antenna elements, provides improved performance characteristics over a single element antenna. The improved characteristics include improved signal to noise ratio, improved interference rejection for received signals, reduced power requirements for transmitted signals, as well as improved directionality.
For an ideal antenna array, the signal characteristics, including attenuation and phase shift, associated with each element of the array are identical. An important goal in designing and manufacturing an antenna array is to optimize the signal characteristics of the array to be as close to ideal as possible. As a result, it is very difficult and expensive to manufacture an antenna array system. Antenna array calibration provides a means for optimizing the signal characteristics of an antenna array using a calibration vector, which is determined based on actual signal characteristics of the array, in order to compensate for performance variances of the actual signal characteristics of each element of the array.
FIG. 1 shows a schematic block diagram of a prior art beam steering antenna array calibration system at 10. The system 10 includes a beam steering antenna array transceiver system 12, including: an antenna array 14 having a plurality of N antenna elements 16; a plurality of N transceivers 18 designated TRANSCEIVER.sub.-- 1 TRANSCEIVER.sub.-- 2, . . . , TRANSCEIVER_N, each of the transceivers 18 having a port 20 communicatively coupled with corresponding one of the antenna elements 16 via a corresponding coaxial cable 22; and a calibration processing unit 24 communicatively coupled with each of the transceivers 18 as further explained below.
Each of the transceivers 18 further includes: a duplexer 30 having a port 32 communicatively coupled with the corresponding one of the antenna elements 16 via port 20 of the transceiver and via the corresponding coaxial cable 22, a receive port 34, and a transmit port 36; a receive processor 38 having an input port 40 communicatively coupled with port 34 of the duplexer, and an output 42; and a transmit processor 44 having a port 46 communicatively coupled with port 36 of the duplexer, and an input port 48. The calibration processing unit 24 includes a plurality of transceiver ports designated TRANSCEIVER_PORT.sub.-- 1, . . . TRANSCEIVER_PORT_N, each of the transceiver ports having an input port 52 for receiving a signal from port 42 of the receive processor 38 of the corresponding one of the transceivers 18, and an output port 54 for providing a signal to port 48 of the transmit processor 44 of the corresponding one of the transceivers.
In operation, the beam steering antenna array transceiver system 12 may be used in any of a variety of applications including a base station for a cellular telephone system. The antenna array 14 receives signals from mobile units, and the controlling processor 24 is operative to analyze the received signals and determine a position vector associated with the corresponding received signal in order to determine the position of the mobile unit. The position vector is then used to control a radiation pattern generated by the antenna array 14 wherein the beam is controlled by varying the phases of signals generated at the output ports 54 of the controlling processor 24 in order to focus the beam in the direction of the corresponding mobile unit.
Each of the antenna elements 16 is associated with a corresponding receive signal path and a corresponding transmit signal path. The receive path associated with each one of the antenna elements 16 extends from the corresponding antenna element 16 to the corresponding input port 52 of the calibration processing unit 24 traversing the corresponding antenna element 16, the corresponding cable 22, the duplexer 30, and the receive processor 38 of the corresponding one of the transceivers 18. The transmit signal path associated with each one of the antenna elements 16 extends from the associated one of the output ports 54 of the calibration processing unit 24 to the corresponding antenna element 16 traversing the corresponding transmit processor 44, duplexer 30, and coaxial cable 22. In an ideal antenna array transceiver system, the signal path characteristics associated with each one of the antenna elements 16 are identical to each other, and the signal characteristics associated with each one of the receiver signal paths are also identical to each other. The signal path characteristics include attenuation, or amplitude difference, in a signal as a result of propagating through a corresponding path, and the phase shift in a signal as a result of propagating through a corresponding path. Therefore, each one of the antenna elements 16 has associated sets of transmit and receive signal characteristics including the phase shift and attenuation associated with the corresponding transmit and receive signal paths. Note that each of the antenna elements themselves may have different signal characteristics associated therewith as a result of very small variances in the dimensions of the antenna elements as well as in the material properties of the corresponding antenna elements.
In practice, an antenna array transceiver system provides less than ideal performance because the signal characteristics of the transmit paths and receive paths associated with each of the antenna elements vary. Therefore, it is necessary to determine the signal characteristics of each of the receive signal paths and each of the transmit signal paths so that calibration compensation values may be determined for each. The calibration compensation values are used to determine a calibration vector which is used to compensate for variances in the signal characteristics associated with each of the transmit signal paths and receive signal paths of the transceiver system. Antenna array calibration provides a means for implementing an antenna array as closely system which provides acceptable performance.
In accordance with conventional processes for calibrating a beam steering directional antenna array transceiver system, either a far-field calibration processor 64 or a transponder 60 may be used to determine a calibration vector for each of a plurality of beam directions determined by positional relationships between the transponder and the array 14. The transponder 60 is responsive to signals transmitted thereto from corresponding ones of the antenna elements 16, and is operative to transmit a return signal back to the antenna array 14. The return signal is received by corresponding ones of the antenna elements 16, and provided to the input ports 52 of the calibration processing unit 24 via the corresponding ones of the coaxial cables 22 and transceivers 18. While either of the external calibration processor 64 or transponder 60 may be used to calibrate the system 12, use of the external calibration processor 64 is complicated because the processor 64 must be controlled either via remote control or manually by a technician in the field.
The object of the calibration process is to determine a compensation vector for use in operation of the system 12 in order to adjust the transmit signals, and receive signals generated and received at ports 52 and 54 of the calibration processing unit 24 in order to compensate for differences between the signal characteristics of each of the transmit and receive signal paths of each of the transceivers 18 and associated elements 16. The calibration process generally includes transmitting and receiving signals between each one of the antenna elements 16 of the array 14 and the transponder 60. The transponder 60 is positioned at a distance far enough away from the antenna array 14 so that the distances between each of the antenna elements 16 is negligible in comparing the signals transmitted and received between the transponder 60 or processor 64 and each corresponding one of the antenna elements 16.
The calibration process includes a receive path calibration process and a transmit path calibration process. In the transmit path calibration process, the calibration processing unit 24 is operative to provide a first reference signal at port 54 of TRANSCEIVER_PORTS.sub.-- 1 to the TRANSCEIVER.sub.-- 1 causing a signal to be radiated from the associated one of the antenna elements 16 to the transponder 60. Next, the calibration processing unit 24 provides a second reference signal at port 54 of TRANSCEIVER_PORTS.sub.-- 2 to the TRANSCEIVER.sub.-- 2 causing a signal to be radiated from the associated one of the antenna elements 16 to the transponder 60. The transponder 60, which receives the signals, may include logic for determining the signal characteristics associated with each signal. Alternatively, the transponder 60 may be coupled via a cable (not shown) to the calibration processing unit 24 which receives data and determines the signal characteristics associated with each of the signals. Based on the signal characteristics associated with each of the signals, a transmit mode calibration vector is determined for each one of the antenna elements.
In the receive path calibration process, the calibration processing unit 24 is responsive to resultant signals received at each of its ports 52, each of the resultant signals being developed at the ports 52 of the processor 24 in response to reference signals generated by the transponder 60 and received by corresponding ones of the elements 16, and propagating through the corresponding one of the cables 22 and transceivers 18. A receive calibration vector is determined by determining amplitude differences and phase shifts between the resultant signals and associated reference signals.
Note that it is necessary in the beam steering process to move the location of the transponder 60, or external calibration processor 64, in order to determine signal characteristics associated with each of the transceivers and corresponding elements for a plurality of beam directions associated with the antenna array 14. The beam must be focused to the position of the transponder.
Another type of antenna array transceiver array system is a smart antenna array transceiver system. Such systems include multi-carrier smart antenna array systems. Unlike traditional beam steering directional antenna array systems which must be calibrated using a far field calibration processor or transponder to determine a calibration director vector for each of the plurality of directions, a smart antenna array system may be calibrated in a different manner. A smart antenna array system is operative to adaptively change the beam direction according to the mobile target direction. A calibration vector provides compensation for variances of the transmit and receive signal paths.
FIG. 2A shows a schematic circuit block diagram of an internal loop calibration system at 80 for calibrating a smart antenna array transceiver system 82. The system 82 includes: an antenna array 14 having a plurality of antenna array elements 16; a plurality of N internal loop calibration transceivers 84 designated TRANSCEIVER.sub.-- 1, TRANSCEIVER.sub.-- 1, . . . TRANSCEIVER_N, each of the transceivers 84 including a port 86 communicatively coupled with a corresponding one of the elements 16 via a corresponding one of a plurality of coaxial cables 88, a reference signal port 90 communicatively coupled with a reference signal terminal 92, a receive signal port 94, and a transmit signal port 96; and a calibration processing unit 100 having a plurality of N sets of transceiver ports each having a corresponding input port 102 communicatively coupled with port 94 of a corresponding one of the transceivers 84, and an output port 104 communicatively coupled with port 96 of the corresponding one of the transceivers 84. A reference signal generator 110, having an output 112, is used to provide a reference signal to each of the terminals 92 in accordance with a prior art "in-loop" calibration process further described below.
FIG. 2B shows a schematic circuit block diagram illustrating further details of one of the internal loop calibration transceivers 84 of FIG. 2A. Each of the transceivers 84 further includes: a first RF signal coupler 122 having a first port 124 communicatively coupled with the corresponding one of the antenna elements 16 via port 86 and via the corresponding coaxial cable 88, a coupling port 126 for receiving the reference signal, or calibration signal, from the reference signal generator 110 (FIG. 2A) via the terminal 92, and a second port 128, a second RF signal coupler 130 having a first port 132 communicatively coupled with port 128 of the first RF signal coupler 122, a coupling port 134, and a second port 136; a duplexer 138 having a port 140 communicatively coupled with port 136 of the second RF signal coupler 130, a transmit port 142, and a receive port 144; a transit processor 146 having an input port 148 communicatively coupled with the corresponding one of the ports 104 of the calibration processing unit 100 via port 96 of the transceiver, and an output port 150 communicatively coupled with the transmit port 142 of the duplexer; an attenuator 152 having an input port 154 communicatively coupled with port 134 of the second RF signal coupler 130, and an output port 156; a switch 160 having a port 162 communicatively coupled with the receive port 144 of the duplexer 138, a port 164 communicatively coupled with port 156 of the attenuator 152, and a port 166; and a receive processor 170 having an input port 172 communicatively coupled with port 166 of the switch 160, and an output port 174 communicatively coupled with the corresponding one of the receive signal ports 102 of the calibration processing unit 100 via port 94 of the transceiver 84.
The switch 160 may be set to connect its port 164 to its port 166, or may be set to connect its port 162 to its port 166 for the purpose of determining transmit calibration vectors and receive calibration vectors as further explained below. The attenuator 152 is also used in the calibration process along with the first and second RF signal couplers 122 and 130 and the reference signal generator 110 (FIG. 2A) which provides the reference signal to terminal 92. Typically, a technician in the field must connect the reference signal generator 110 (FIG. 2A) to each of the reference signal terminals 92 of the transceivers 84 in succession during the prior art calibration process which is a laborious task.
In a receiver calibration mode, switch 160 is set to couple the receive port 144 of the duplexer 138 to the input port 172 of the receive processor 170 by connecting ports 162 and 166 of the switch Also in the receive calibration mode, the corresponding one of the coaxial cables 88 is disconnected from the corresponding antenna element 16, and the cable is terminated in order to isolate the corresponding antenna element from the transceiver. Further, in the receive calibration mode, the signal generator 110 (FIG. 2A) is connected to the corresponding terminal 92 and activated to provide a reference signal to the coupling port 126 of the first RF signal coupler 122. The object of the prior art receive calibration process is to determine signal characteristics associated with a tested receive signal path 180 extending from the coupling port 126 of the first RF signal coupler 122 to the input port 102 of the processing unit 100 via ports 126 and 128 of the first RF signal coupler 122, ports 132 and 136 of the second RF signal coupler 130, ports 140 and 144 of the duplexer 138, ports 162 and 166 of the switch 160, and the receive processor 170.
By applying the reference signal to the terminal 92 while the switch 160 is set in the receive mode and while the cable 88 is terminated as described above, a receive calibration mode resultant signal is developed at port 174 of the receive processor 170 as a result of the reference signal propagating through the tested receive signal path 180. The calibration processing unit 100 is responsive to the receive mode calibration resultant signal received at its port 102 from port 174 of the receive processor 170, and operative to determine signal characteristics associated with the tested receive signal path 180 based on an amplitude difference and phase shift between the reference signal applied to terminal 92 and the receive mode calibration resultant signal. In accordance with this prior art method, it is assumed that the signal characteristics of the tested receive signal path 180 adequately represent the signal characteristics of an actual receive path which extends from the associated antenna element 16 to the signal path characteristics of the associated input port 102 of the processing unit 100 via the associated one of the cables 88, ports 124 and 128 of the first RF signal coupler 122, ports 132 and 136 of the second RF signal coupler 130, ports 140 and 144 of the duplexer 138, ports 162 and 166 of the switch 160, and the receive processor 170. An important problem associated with the prior art internal loop calibration process is that the signal characteristics associated with the tested receive signal path 180 do not include the signal characteristics associated with the antenna element 16, and the associated one of the coaxial cables 88 because these elements are bypassed by the injection of the reference signal at terminal 92 which is injected at the coupling port 126 of the first RF signal coupler 122. Therefore, the described prior art calibration process does not account for differences in the signal characteristics associated with each of the antenna elements 16, each of the coaxial cables 88, and the path between ports 124 and 128 of each of the couplers 122.
Another problem associated with the prior art internal loop calibration process is that the switch 160, attenuator 152, and RF signal couplers 122 and 130 introduce a significant amount of attenuation in the receive signal path of the transceiver 84 which reduces the sensitivity of the antenna system. Yet another problem associated with the prior art internal loop system is that it is assumed that the attenuator 152 has a precisely known attenuation value, while in practice the attenuation value of the attenuator 152 may vary.
In a transmit calibration mode, the switch 160 is set to couple port 156 of the attenuator 152 to port 172 of the receive processor 170 by connecting ports 164 and 166 of the switch The prior art transmit mode calibration process requires measuring signal characteristics of two signals paths in accordance with a two step process as further explained below.
In accordance with a first step of the prior art internal loop transmit mode calibration process, the calibration processing unit 100 generates reference signals at each of its ports 104, each of the reference signals having a known phase and amplitude. The reference signal generated at each of the ports 104 propagates through a loop signal path 182 traversing the transmit processor 146, ports 142 and 140 of the duplexer 138, ports 136 and 134 of the second RF signal coupler 130, the attenuator 152, ports 164 and 166 of the switch 160, and the receive processor 170. The calibration processing unit 100 is responsive to a first transmit mode resultant signal received at its port 102, the first transfer mode resultant signal being developed at the output port 174 of the receive processor as a result of the reference signal, generated at the corresponding port 104, propagating through the loop signal path 182. The calibration processing unit 100 is operative to compare the resultant signal received at port 102 to the associated reference signal generated at the corresponding one of the ports 104 which has a known phase and amplitude. The calibration processing unit 100 is further operative to determine the signal characteristics associated with the signal path 182. The signal characteristics associated with the signal path 182 of each of the transceivers 84 (FIG. 2A) are used to determine a vector X which is used to determine a transmit mode calibration vector as further explained below.
In accordance with a second step of the prior art internal loop transmit mode calibration process, the signal characteristics associated with a residual signal path 184 must be measured. The residual signal path 184 extends from port 126 of the first RF signal coupler 122 to port 102 of the calibration processing unit 100 and transfers ports 126 and 128 of the first RF signal coupler, ports 132 and 134 of the second RF signal coupler 130, the attenuator 152, ports 164 and 166 of the switch 160, and the receive processor 170. The switch 160 is set to communicatively couple port 156 of the attenuator 152 with port 172 of the receive processor 170 by connecting ports 164 and 166 of the switch. A second reference signal, having a known phase and amplitude, is then applied to the reference signal terminal 92 using the reference signal generator 110 (FIG. 2A). The calibration processing unit 100 is responsive to a second resultant signal received at its port 102, and operative to determine the signal characteristics associated with the signal path 184 by determining a phase shift and amplitude difference between the reference signal provided to the reference signal terminal 92 and the second resultant signal which is developed as a result of the reference signal propagating through the signal path 184. The signal characteristics associated with each path 184 of the transceivers 84 (FIG. 2A) are used to determine a vector Y.
A transmit calibration vector associated with a tested transmit signal path may be determined in accordance with relationship (1), below. EQU Transmit calibration vector=X.cndot./Y (1)
Wherein the vector Y represents the signal characteristics associated with each of the residual paths 184 of the transceivers 84 (FIG. 2A), and wherein the vector X represents the signal characteristics associated with each of the loop signal paths 182 of the transceivers 84 (FIG. 2A). Relationship (1), above, yields a transmit calibration vector that is determined by considering the signal characteristics associated with a tested transmit signal path which extends from port 126 of the first RF signal coupler 122 to port 174 of the receive processor 170 via ports 126 and 128 of the first RF signal coupler, ports 132 and 136 of the second RF signal coupler 130, ports 140 and 144 of the duplexer 138, ports 162 and 166 of the switch 160, and the receive processor 170.
Another important problem associated with the prior art internal loop calibration process is that the signal characteristics associated with the tested transmit signal path do not include the signal characteristics associated with the antenna element 16, and the associated one of the coaxial cables 88 because these elements are bypassed by the injection of the reference signal at terminal 92 which is injected at the coupling port 126 of the first RF signal coupler 122. The signal characteristics associated with each of the elements 16 are significant because the radiation from each of the elements 16 is different as each of the elements 16 has different signal characteristics including slightly different dimensions and slightly different material properties.